Pipes (so called Oil Country Tubular Goods (OCTG)) used for oil fields or natural gas fields have a unit length of ten or more meters. The pipes are connected to each other by the treaded connections, and the connected pipes (the connected oil country tubular goods) have an overall length as long as several thousand meters.
Threaded connections for pipes or tubes are classified into T&C (threaded and coupled) type threaded connections and integral type threaded connections.
A T&C type threaded connection includes two pins formed at each end of two pipes or two tubes, and two boxes formed at both ends of a coupling that is a short tube and has an outer diameter larger than the pipes or tubes. Each pin has an outer surface having male screws thereon. Each box has an inner surface having female screws thereon. Each pin is screwed into each box to be fastened thereto. Specifically, in a T&C type threaded connection, the pipes are connected to each other via the coupling.
Meanwhile, an integral type threaded connection includes a box formed at an end of a first pipe, and a pin formed at an end of a second pipe. The pin of the second pipe is screwed into the box of the first pipe, thereby connecting the first and the second pipes to each other. This means that in the integral type threaded connection, the first and the second pipes are directly connected to each other. A coupling is eliminated by using an integral type threaded connection. Hence, there is no outward extrusion by a thickness of the coupling, and thus there is no interfere with an inner surface of a pipe located outward. Accordingly, integral type threaded connections are used for a special usage such as horizontal excavations.
In general, threaded connections are required to have endurance against a tensile forth in the axial direction due to their own weights of the connected pipes as well as endurance against pressures of external and internal liquids.
Threaded connections are further required to have galling resistance. Specifically, a preferable galling resistance is required even after repetitive use four or more times in a casing pipe (large-diameter size), and ten or more times in a tubing pipe (small-diameter size). Conventionally, in order to enhance the galling resistance, copper plating films are formed, or surface treatments, such as phosphatizing, are applied on contact surfaces of pins or boxes of threaded connections. A contact surface denotes a surface portion where a pin and a box come into contact with each other, and such a contact surface includes a threaded portion that is threaded, and a non-threaded metal contact portion that is not threaded. A seal portion is equivalent to the non-threaded metal contact portion.
For the purpose of enhancing the galling resistance, prior to fastening, dope is applied on the contact surface of the pin or the box. The dope is a compound grease containing heavy metals, such as Pb.
However, heavy metals may affect the environment, and usage of dope containing heavy metals has been increasingly restricted. For this reason, dope (referred to as “green dope”) free from heavy metals, such as Pb, Zn, and Cu, has recently been developed. However, green dope has a lower galling resistance than that of conventional dope.
As techniques to enhance the galling resistance without using dope, there have been proposed 1) a method of dispersingly mixing fluororesin particles in a plating film, 2) a method of forming a lubricating protective film through spattering, and 3) a method of using a solid lubricating film instead of using a compound grease, and other methods. However, each of these techniques provides a poorer galling resistance compared with that of conventional dope.
Japanese Patent Application Publication No. 2003-74763 (Patent Literature 1) and Japanese Patent Application Publication No. 2008-215473 (Patent Literature 2) propose threaded connections excellent in galling resistance. In Patent Literature 1, a Cu—Sn alloy layer is formed on a threaded portion and a non-threaded metal contact portion of a threaded connection. In addition, in Patent Literature 2, a Cu—Zn-M1 alloy layer (M1 is one or more types of elements selected from Sn, Bi, and In) is formed on a threaded portion and a non-threaded metal contact portion.